The present invention generally relates to an optical transducer system for measuring the rotational speed of a wheel. The present invention specifically relates to a double interrupter disc, single optical fiber, reflective wheel speed transducer system.
The prior art includes several different types of optical transducers for measuring the rotational speed of a rotating object. Most of these transducers are transmissive systems, that is, the light source and light receiver are placed on opposite sides of a rotating interrupter disc. The interrupter disc modulates the light transmitted from the source towards the receiver at a rate proportional to the angular velocity of the rotating interrupter. The rate of light modulation can then be decoded to produce an electrical signal proportional to the angular velocity of the interrupter, or an external wheel which drives the interrupter. U.S. Pat. No. 3,559,065 (Grundy) discloses one such transmissive optical transducer.
U.S. Pat. No. 3,698,772 (Nixon) discloses a reflective optical transducer system. Nixon provides a reflective interrupter disc, which is driven at an angular velocity that is a function of the rotational speed of a wheel. The disc carries a series of light absorbing, radial markings. A light source is located adjacent the disc to reflect light toward the input of a photocell. As the disc rotates, the level of light reflected from the disc varies to provide a variable output signal from the photocell. Electrical decoding circuitry converts the variable output signal into a wheel speed signal.
Both the Grundy transmissive system and the Nixon reflective system have several drawbacks. First, neither system uses fiber optics. Optical fibers would allow the electrical elements of the light source and decoding circuitry to be remotely located from the interrupter disc. The interrupter disc is usually located in an electrically noisy environment. Thus, fiber optics can provide a means for isolating electrical components from an electrically noisy environment.
Second, if either Grundy or Nixon were provided with fiber optics, each system would require at least two optical fibers--one from the light source to the interrupter, and one from the photocell to the decoding circuitry. Optical fibers on both sides of the interrupter can create transducer design problems. Space constraints in a transducer often require the electrical light source circuitry and decoding circuitry to be located in the same vicinity. If one optical fiber leads directly from the light source to the interrupter, the other fiber must travel back around the interrupter, without interfering with it, to the decoding circuitry. This requirement complicates the transducer design.
Third, Nixon does not disclose a means for indicating circuit failure and for monitoring system integrity. If Nixon's light source failed, for example, extraneous light in the interrupter area could still produce wheel speed signals. Grundy discloses a means for indicating circuit failure and for monitoring system integrity, but Grundy requires a pulsating light source. A carrier detector is provided at the photocell output to detect the light pulse frequency. The absence of the pulse frequency indicates some malfunction in the transducer. While Grundy's system will detect circuit failures, it complicates the system by requiring light pulse generation and detection circuitry.
Fourth, both Grundy and Nixon use only one interrupter disc. In a single disc system, the light modulation signal is basically attained by counting lines. As a result, the line spacing must be similar to the diameter of the light beam. This has the effect of limiting the measurement resolution of rotation frequency.